Supported Pd nanoparticles constitute the active phase in catalysts used for energy conversion, chemical synthesis, and pollution abatement. To achieve the highest selectivity and reactivity, it is desirable to have well-dispersed nanoparticles (NPs) that have identical properties and distribution of active sites [1]. Unfortunately, conventional synthesis routes do not provide the requisite degree of control, since they start from Pd salts that are first deposited on a support by impregnation or precipitation. The impregnation method is simple but time-consuming, as it is often necessary to coat the support multiple times. Each time the solvent must be dried before repeating the process, which can take up to several hours per iteration. Once the desired amount of precursor has been coated, the powder is then oxidized and reduced to produce nanoparticles with uncontrolled size and dispersion over the support. The precipitation method begins with the desired salts in an acidic solution and adding a base to precipitate a solid, which is dried, calcined, and reduced. In both the impregnation and precipitation methods, the resulting nanoparticles are generally unevenly dispersed over the support and exhibit a broad size distribution. Furthermore, reduction is achieved by high-temperature treatments involving calcination and H2 reduction, or chemical reduction by sodium borohydride [2]. The resulting broad distribution of particle size, shape, and composition is detrimental to catalyst performance. Hence, there has been considerable interest in developing colloidal routes to synthesize well-defined nanoparticles that could be used to prepare heterogeneous catalysts [3-5]. Typically, solution routes require various reducing agents such as hydrazine [6], alkaline borohydrides, [7], or amine groups [8] where the particles are protected by polymer groups, surfactants or ligands to prevent agglomeration and growth [7,9].
Polymer protecting agents such as poly(vinyl pyrrolidone) (PVP) and polyvinyl alcohol (PVA) allow preparation of metal colloids that can be stable for months with reasonable control over size as well as shape [10-14]. The synthesis involves addition of polymer to the metal salt followed by chemical or thermal reduction to produce a stable black suspension of Pd0 particles. These polymer-capped nanoparticles have been shown to be capable of adsorbing probe molecules (i.e., CO) [12] and to be active in a variety of liquid-phase reactions, such as olefin hydrogenation [3] as well as alcohol oxidation [15]. However, when the polymer-capped nanoparticles are deposited on a support for gas-phase reactions, the capping agent must be removed to achieve catalytic activity [1,16,17]. The high-temperature oxidation and reduction treatments used can lead to particle growth and loss of monodispersity. Therefore, there is a need to develop novel routes that can provide metal nanoparticles without protective polymers or capping ligands.
In solvothermal synthesis, precursors, such as palladium chloride or palladium (bis acetyl acetonate), are added to high boiling solvents such as bromobenzene, toluene, or methyl isobutyl ketone in the presence of a surfactant to achieve reduction of the metal [18]. A more easily reduced precursor, such as palladium acetate (noted as Pd(OAc)2) allows for colloidal synthesis at lower temperature using simple alcohols as reducing agents [4,5,19]. However, literature reports that utilize methanol (MeOH) without a capping agent indicate that large aggregates will form [19]. These aggregates can reach diameters of 50 nm and are not suited for catalytic applications. The uncontrolled reduction in Pd complexes at elevated temperatures has been described in the homogeneous catalysis literature as a nuisance [20]. These studies all suggest that capping agents or ligands are essential for the synthesis of nanoparticles in solution at elevated temperatures.
Recent work by Chen et al [21] has shown that graphene oxide can directly reduce K2PdCl4 to produce NPs. While this is an effective technique and yields Pd nanoparticles of about 3 nm in diameter, it is limited to reactions using graphene oxide as a support.